Friction Properties of Tribofilm Formed From Engine Oil Additives (Part 2): Frictional and Mechanical Properties in nm-Scale and Mechanisms of Friction Reduction

摘要

Nanometer-scale mechanical and frictional properties in depth and near the surface of MoDTC/ZDDP and ZDDP tribofilms were determined by nanoindentation and nanoscratch techniques combined with in-situ atomic force microscopy (AFM) observation. Both types of tribofilm were found to be friction-functionally graded materials with the same frictional and mechanical property depth distributions except in the area just below the surface. Their friction coefficients decrease from 0.35 to the low value around 0.16 for lubrication with a decrease in the depth ranging from 60 nm to 10 nm. Both the tribofilms were found to possess a low friction inner-skin layer just below the film surface at a depth of around 10 nm. Most importantly, the inner-skin layer of the MoDTC/ZDDP triboflm is much thinner and possesses lower shearing yield stress and lower friction coefficient than that of the ZDDP tribofilm. The results of this study suggest that the inner-skin layer of the MoDTC/ZDDP tribofilm is constructed of a few S-Mo-S sheets.